In the retorting of crushed oil shale with hot eduction gases, a persistent problem has been that of providing efficient retorting of shale particles varying widely in size. Commonly applied shale crushing techniques are controlled to produce a consist containing no particles larger than a permissible maximum, usually about 2-4 inches in diameter. Such crushing also inherently produces from about 2-15 wt.% of shale "fines" having a diameter of less than about 1/4 inch. If this entire consist is utilized in the most prevalent of retorting methods; i.e. countercurrent flow of hot retorting gas with a compact moving bed of the shale particles, serious problems arise. Among these problems are high gas pressure drop through the bed, uneven distribution of gas flow in the bed and increased solids content in the product oil.
To avoid these problems, it has become customary in the art to screen out the fines from the retort feed, and either subject them to separate retorting, or discard them. It would obviously be economically attractive to provide some means for integrating the retorting of shale fines with the retorting of the main body of shale, if the aforementioned problems could be avoided.
It has now been found that such an integration can be achieved in a particular type of retorting in which the main body of crushed shale is pumped upwardly through a retort, countercurrent to preheated downflowing eduction gas, and spent shale above the top of the retort is allowed to form a free standing cone with an angle of repose permitting the free gravitation of spent shale from the apex of the cone downwardly and outwardly over the upper edge of the retort. According to my invention, in such retorting up to about 15 wt.%, based on total shale feed to the retort, of fines having a diameter of about 1/4 inch or less can be distributed on the top surface of said cone of spent shale, at or near the apex thereof, and the resulting residence time of such fines under undisturbed gravity flow conditions is sufficient to effect substantially complete retorting thereof with no significant adverse effects on pressure drop, gas flow distribution, or solids content of the product oil which is taken off at the bottom of the retort.